Process for manufacturing monolithic multilayer piezoelectric actuator

ABSTRACT

A monolithic piezoelectric actuator is made of a stoichiometric PZT ceramic ( 2 ) with low A-site doping, and electrode layers ( 1 ) containing silver and palladium. It exhibits an improved mechanical strength with good piezoelectric properties. The production process leads to optimum grain sizes and optimum piezoelectric properties irrespective of any B-site doping in the ceramic. Multilayer piezoelectric actuators with high application temperatures up to 150° C. can be obtained.

BACKGROUND OF THE INVENTION

Piezoelectric actuators normally comprise a plurality of piezoelectricelements arranged in a stack. Each of these elements in turn comprises apiezoceramic layer which is provided on both sides with metallicelectrodes. If a voltage is applied to these electrodes, then thepiezoceramic layer reacts with a lattice distortion which leads to ausable lengthwise expansion along a major axis. Since this in turnamounts to less than two parts per thousand of the layer thickness alongthe major axis, a correspondingly higher layer thickness of activepiezoceramic must be provided in order to achieve a desired absolutelengthwise expansion. With increasing layer thickness of thepiezoceramic layer within one piezoelectric element, however, thevoltage necessary for the response of the piezoelectric element alsorises. In order to keep this within manageable limits, the thicknessesof individual piezoelectric elements in multilayer actuators normallylie between 20 and 200 μm. A piezoelectric actuator must therefore havean appropriate number of individual elements or layers for a desiredlengthwise expansion.

Known piezoelectric actuators of multilayer design therefore comprise upto several hundred individual layers. These can be arranged to form astack and, for example, can be adhesively bonded. U.S. Pat. No.5,438,232 discloses a process for the production of multilayer actuatorsby bonding individual actuators with the aid of a resin. However, such abonded stack exhibits too low a stiffness for many applications, inparticular when high forces have to be transmitted using thepiezoelectric actuator. Sufficiently high stiffnesses are possessed bypiezoelectric actuators of monolithic multilayer design. In order toproduce them, piezoceramic green films are arranged alternately withelectrode material to form a stack and are sintered together. Only inthis way is it to achieve a sufficiently solid composite of theindividual layers in the stack. An article by H. Moilanen et al. in thejournal Sensors and Actuators A , 43 (1994) 357 to 365 discloses aprocess for the production of a multilayer piezoelectric actuator inwhich both the ceramic layers and the electrode layers are produced byalternating overprinting. In this case, drying or presintering attemperatures up to 750° C. is necessary at regular intervals.

An article by S. Takahashi et al. in Ferroelectrics, 1983, Vol. 90,pages 181 to 190, discloses a process for the production of a multilayeractuator which is obtained by stacking ceramic green films printed withelectrode layers on one another and laminating them, and subsequentsintering of the-stack.

In the production of monolithic multilayer piezoelectric actuators, thematerial properties both of the piezoceramic and of the electrodematerial must be taken into account during the setting of the processconditions, in particular during the sintering process. Problems areposed, for example, by the optimum sintering temperature forpiezoceramic, which, in order to achieve optimum grain sizes and henceoptimum piezoelectric properties as a function of the composition of thepiezoceramic, may lie above 1250° C. At such a high sinteringtemperature, only platinum can be used as the electrode material. Thisexhibits a weak interaction with the ceramic and can be used togetherwith most piezoceramic materials. However, the high material costs forplatinum are disadvantageous, as is the limited strength at theinterface between electrode and piezoceramic.

If Ag/Pd, which is cost-effective and common in multilayer capacitors,is used as the electrode material, then the sintering temperature islimited by the melting point of the alloy, which may, for example, liebelow 1130° C. (in the case of Ag/Pd 70/30). Hence, one is limited topiezoceramic materials whose optimum sintering temperature lies at mostat the melting point of the alloy. To lower the optimum sinteringtemperature, such piezoceramics contain B-site dopings of typically 20to 50 percent in relation to the lead zirconium titanate (PZT) basicmaterial. As a disadvantage, in the case of these ceramics a loweredCurie temperature must be tolerated, which limits the maximumapplication temperature of the piezoelectric actuator. In addition, inthe case of this material combination there has also been shown to be alimited strength in the stack at the piezoceramic/electrode interface.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to specify aproduction process for a piezoelectric actuator of monolithic multilayerdesign which is not limited to piezoceramic material of low Curie andapplication temperature, which nevertheless has good piezoelectricproperties and which additionally possesses a high mechanical compositestrength.

According to the invention, this object is achieved with a process inaccordance with claim 1. Special refinements of the invention emergefrom further claims.

In general terms the present invention is a process for producing apiezoelectric actuator of monolithic multilayer design. To producepiezoceramic green films, the starting point is a stoichiometricpiezoceramic powder of the PZT type, to which are added a stoichiometricexcess of a heterovalent rare earth metal up to an overall content of 1to 5 mol % and a stoichiometric excess of an additional 1-5 mol % oflead oxide. Electrode layers of a paste containing silver and palladiumare applied to the green films. The green films are stacked on oneanother and then laminated such that an alternating sequence results ofgreen films and electrode layers in the stack. The laminated stack issintered under controlled sintering conditions such that excess leadoxide evaporates off and the hyperstoichiometric rare earth doping iscompensated by inward diffusion of silver from the electrode layers. Thesintering is carried out at a maximum temperature of 1130° C. in anoxidizing atmosphere. During the sintering, a holding phase of 30 to 120minutes at the maximum temperature is maintained. Stoichiometricpiezoceramic layers with homogeneous silver doping are thereby obtained.

Advantageous developments of the present invention are as follows.

For a desired thickness of the electrode layers of 2-4 μm (followingsintering), at the time of applying the electrode layers a higher layerthickness is selected as a layer thickness allowance which compensatesthe subsequent layer thickness loss by the inward diffusion of silverinto the piezoceramic layer. The layer thickness allowance is determinedin proportion to the layer thickness ratio of piezoceramic layer toelectrode and to the dopant content of the rare earth metal.

As the rare earth metal, La or Nd in stoichiometric excess is added tothe piezoceramic powder.

A piezoceramic powder is used which has complex B-site doping.

With the invention, for the first time a piezoelectric actuator isproduced which has optimum grain sizes in the piezoceramic layers,irrespective of any doping which may be present on a B site with anelectrode layer containing silver/palladium. The piezoelectric actuatorspossess the optimum values which are known and expected from apiezoceramic layer of identical composition which is sintered underoptimum conditions and separately from the electrode layer. In thiscase, the piezoelectric actuator has a monolithic design in whichpiezoceramic green films and electrode layers have been sinteredtogether and therefore have a high strength in the connection betweenelectrode layer and ceramic layer. The electrode layer, which consists,for example, of a silver/palladium alloy, remains undamaged during thesintering process, since the latter can be carried out below the meltingtemperature of the electrode material. It is particularly surprisingthat in this way it is also possible to obtain a piezoelectric actuatorwhich combines a low or entirely absent B-site doping, cost-effectivesilver/palladium electrode layers, high grain sizes and goodpiezoelectric properties of the piezoceramic layers with simultaneoushigh composite strength of the individual layers in the stack. In thisway it is possible to obtain, in particular, even piezoelectricactuators which possess a high Curie temperature which enables use ofthe piezoelectric actuator at relatively high operating temperatures.This was previously not known, since piezoceramics with high Curietemperatures require higher sintering temperatures than were previouslypossible with the low melting point of the electrode layers used.Non-optimum sintering conditions at too low a sintering temperaturehamper the grain growth, however, and yield piezoceramics with poorpiezoelectric properties. Good piezoelectric properties, on the otherhand, are obtained if, as in the case of the piezoelectric actuatoraccording to the invention, the grain sizes of the piezoceramic layerslie in the same range from 2 to 10 μm as is achieved in the case ofsintering the ceramic on its own, that is to say without electrodes,using a higher optimum sintering temperature.

For the production of the piezoelectric actuator, likewise according tothe invention, a starting point is a known process for the production ofmonolithic multilayer components, in which piezoceramic green filmsprovided with electrode material are alternately stacked on one anotherand are then sintered together. According to the invention, the startingpoint is a piezoceramic powder of the lead zirconate titanate (PZT)type, which has a stoichiometric composition. In addition to thisstoichiometric composition, a small proportion of a heterovalent A-sitedopant at the level of 1 to 5 mol percent as well as a further excess ofan additional 1 to 5 mol percent lead oxide are added. In addition, anelectrode material containing silver and palladium is used. The stack isthen laminated and sintered under controlled conditions in an oxidizingatmosphere, the sintering temperature being allowed to reach at most themelting point of the electrode material containing silver and palladium.The sintering is carried out such that stoichiometrically excess leadoxide evaporates off and that, in addition, silver diffuses out of theelectrode layers into the piezoceramic layers, a stoichiometricpiezoceramic composition being obtained.

The invention is based on the surprising knowledge that ahyperstoichiometric A-site doping, for example by a higher-valent rareearth metal, can be compensated during the sintering process by inwarddiffusion of silver from the electrode layers. However, the preconditionfor this is that at the same time a stoichiometric excess of lead oxideis present. As an explanation for this, it is presumed that, during thesintering process, excess lead oxide produces liquid phases within thedeveloping piezoceramic structure, which phases promote the inwarddiffusion of silver. It is furthermore surprising that the diffusionprocess of the silver is, so to speak, self-regulating. The drivingforce for the diffusion process is the heterovalent defect population ofthe A sites in the PZT crystal lattice, which are compensated by themonovalent silver. When a stoichiometric composition is reached, thedriving force falls away, with the result that further inward diffusionof silver ceases. Furthermore, it is surprising about the process that,in spite of a maximum sintering temperature of, for example, 1130° C.for a 70/30 silver/palladium alloy, high grain sizes in the piezoceramicare obtained. It is presumed that the incorporation of the silver intothe piezoceramic promotes the grain growth. In spite of a sinteringtemperature which. is reduced by 150 to 200° , just as high grain sizesand good piezoelectric values are achieved as in the case of a componentsintered under optimum conditions and without electrodes, and for whichcorrespondingly higher sintering temperatures can be selected. Theachievement of an optimum density of at least 96 percent is also shiftedto lower temperatures.

As a result of the excess of lead oxide set in piezoceramic powder, afurther advantageous result is achieved. At the piezoceramic/electrodelayer interface, a phase containing Pd-Pb-O forms as a result of inwarddiffusion of palladium, this phase being able to be detected in theinterface region in grain boundary interstices following sintering. Itis presumed that these phases are responsible for the improved adherencewhich has been established between the ceramic layers and electrodelayers in the stack.

In an advantageous way, the sintering is carried out in an oxidizingatmosphere. Once the maximum sintering temperature has been reached, aholding phase of 30 to 120 minutes is maintained at this maximumtemperature.

Both the lead excess and the excess A-site doping are preferably set to1 to 3 mol percent independently of each other. A rare earth, preferablylanthanum or neodymium, is selected for the A-site doping.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which.

FIG. 1 shows a schematic cross-section through an alternatingpiezoceramic/electrode stack following sintering.

FIG. 2 shows a measured curve for the silver content of a piezoceramiclayer in relation to the layer thickness following sintering.

FIG. 3 shows in the form of a detail a region in the vicinity of theelectrode layer/piezoceramic layer interface following sintering.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Production of a piezoelectric actuator from a piezoceramic having ahigh Curie temperature of 330° C.

A piezoceramic powder is prepared which has a nominal composition ofPb_(0.98) Nd_(0.02) (Zr_(0.54) Ti_(0.46)) O_(3.01). The startingmaterials, mixed as homogeneously as possible, can be produced in aaccordance with known processes and be constituted, for example, inaccordance with the mixed oxide process or via chemical routes, forexample in accordance with the sol-gel process, the citrate process, theoxalate process or via other organometallic precursor compounds. Whereasfor the mixed oxide process all the cations provided for the ceramic aremixed with one another in the form of their oxides and are thenconverted into PZT, other production processes start from mixedsolutions of organometallic compounds of the desired cations. By meansof precipitation from solution or by means of gradual thickening in theso-called solgel process, an extremely homogeneous distribution of thecations in the subsequent solid is achieved.

Following calcining, the product is re-ground, homogenized and thenmixed with an organic binder. Green films are then drawn or cast usingthe slip obtained in this way. Following drying of the green films,these are provided with electrode material, for example printed with apaste which contains particles of a silver/ palladium alloy (70/30 massratio) in a binder with an overall printable consistency.

The piezoceramic green films are produced in a thickness which, takinginto account linear shrinkage during the sintering of typically 15percent, yields a piezoceramic thickness of 20 to 200 μm. For theelectrode layer, sufficient electrode material is printed on to yield anelectrode layer of about 2 to 3 μm thickness after the sintering. Givena small layer thickness ratio between the electrode layer andpiezoceramic layer, correspondingly more electrode material must beprinted on in order that the process of inward silver diffusion, leadingto an electrode material loss, can be compensated. In this case theelectrode layer can be printed on over the entire surface or in anydesired but as fine as possible pattern.

The piezoceramic green films, printed with electrode material, are thenstacked on one another, an alternating arrangement of piezoceramic layerand electrode layer resulting. By means of a laminating process underpressure and elevated temperature, the stack is precompressed andthereafter already exhibits sufficient coherence so that it can behandled as a stack. In this stage, it is also possible to divide a stackof relatively large base area, following lamination, into a plurality ofidentical stacks of smaller base area, for example by means of cuttingor stamping. A plurality of such part-stacks can in turn be combined toform a larger stack. The total number of the layers which is requiredfor the subsequent piezoelectric actuator depends on the level ofdeflection which is intended to be achieved using the piezoelectricactuator by means of applying a voltage. Since an individualpiezoelectric element can be deflected typically by 1 to 2 parts perthousand of its thickness, the required number of individual layers fora desired total displacement can be quite simply calculated via thetotal layer thickness of the piezoceramic layers (following sintering).For example, for a desired application with a displacement of 20 μm,about 150 individual piezoceramic layers of about 100 μm thickness aresufficient.

Following the lamination of the stack or stacks, sintering takes placein an oxidizing atmosphere at 1130° C. This maximum temperature ismaintained for about 1 hour and is then cooled down slowly.

FIG. 1: a piezoelectric actuator is obtained which has a high mechanicalcomposite strength and therefore a high mechanical loadability. As canbe shown by means of transverse sections through the stack, theelectrode layers 1 are inherently largely coherent. A high degree ofarea coverage is thus achieved, which allows a homogeneous electricfield when a voltage is applied. The piezoceramic layers 2 have highgrain sizes of 2 μm to 10 μm. An analysis of the piezoceramic yields thefollowing composition: Pb_(0.96) Ag_(0.02) Nd_(0.02) (Zr_(0.54)Ti_(0.46)) O₃. The composition is not only stoichiometric but alsohomogeneous over the entire piezoceramic layer. This can be verified, inparticular, by a measurement of the silver concentration by means ofmicroanalysis.

FIG. 2 shows the profile of the silver content as a function of thedistance from the piezoceramic layer/electrode layer interface. It canbe seen that the silver concentration is extremely homogeneous over theentire layer thickness.

FIG. 3 shows, in an enlarged schematic cross-sectional representation,the electrode layer/piezoceramic layer interface area. The granulation 3of the piezoceramic layer can be seen well. The grain boundaryinterstices 4 at the interface to the electrode layer 1, that is to saythe interspaces geometrically predefined by the shape of the ceramicgrains, exhibit a phase in which palladium, lead and oxygen can bedetected. This phase can be detected up to a distance of 50 nm to a fewμm from the interface to the electrode layer. It is assumed that, theelectrode layer 1 interengages with the piezoceramic layer 2 with theaid of this phase and contributes to their increased strength accordingto the invention. Deeper within the piezoceramic layer or, for example,in the piezoceramic grains 3, no palladium can be detected.

The piezoelectric actuator of this composition is suitable forapplication temperatures up to about 150° C.

2. Production of a piezoceramic layer with low Curie temperature of 170°C.

A piezoceramic powder of the nominal composition Pb_(0.99) La_(0.01){Zr_(0.30) Ti_(0.36) (Ni_(⅓) Nb_(⅔))_(0.34)}O_(3.005) is produced inaccordance with known methods. Corresponding to the first exemplaryembodiment, green films are prepared therefrom, provided with electrodematerial, layered to form the corresponding stacks, laminated andsintered under identical conditions. A piezoelectric actuator of highstrength is obtained, for which a Curie temperature of 170° C. isdetermined. This actuator can therefore be used in a temperature rangeup to a maximum of about 80° C. The piezoceramic layers 2 have thefollowing stoichiometric composition after the sintering: Pb_(0.98)Ag_(0.01) La_(0.01) {Zr_(0.30) Ti_(0.36) (Ni_(⅓) Nb_(⅔))_(0.34)}O₃.

This piezoceramic or, respectively, the piezoelectric actuator ofmultilayer design produced from it, possesses a complex doping for the Bsite, as can be seen from the formula. As a result, a piezoceramic withimproved piezoelectric properties is obtained which exhibits inparticular an increased relative deflection.

Added to these improvements in properties, known per se, as a propertyof the invention, is the fact that an improved strength in the compositeof the piezoelectric actuator is also exhibited here. This ceramiccomposition is also stoichiometric following sintering, since the excess(La) doping is compensated during sintering by inward diffusion ofsilver, and excess lead oxide escapes by evaporating out of the ceramic.Here, too, a uniform silver content over the ceramic layer results,while palladium can once more only be detected in areas close to theinterface in the form of the phases containing Pd-Pb-O in the grainboundary interstices 4 already mentioned in the case of the firstexemplary embodiment.

With the invention it is possible to constitute a monolithicpiezoelectric actuator of multilayer design which, by comparison withknown piezoelectric actuators, exhibits an improved composite strength,can be produced with cost-effective electrode layers containing silver/palladium, and, completely independent of any B-site doping which may bepresent, can also be sintered below an intrinsically optimum sinteringtemperature to form a high-quality ceramic having good piezoelectricproperties. Therefore it is in particular possible to produce multilayeractuators having application temperatures up to 150° C., high mechanicalloadability and high reliability, even in dynamic operation.

The invention is not limited to the particular details of the methoddepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described method withoutdeparting from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A process of providing a piezoelectric actuatorof a monolithic multilayer design, comprising the steps of: providingpiezoceramic green films that are of a stoichiometric piezoceramicpowder of a lead zirconium titanium type (PZT), said films having astoichiometric excess of a heterovalent rare earth metal up to anoverall content of 1 to 5 mol% and a stoichiometric excess of anadditional 1 to 5 mol% of lead oxide; applying electrode layers of apaste containing silver and palladium to the green films; stacking thegreen films on one another to form a stack of films, laminating thestack of green films so that an alternating sequence of green films andelectrode layers results in a laminated stack; and sintering thelaminated stack under controlled sintering conditions of an oxidizingatmosphere at a maximum temperature of 1130° C. for a period of time ina range of 30 to 120 minutes so that excess lead oxide evaporates off, athe hyperstoichiometric rare earth doping is compensated by inwarddiffusion of silver from the electrode layers and stoichiometricpiezoceramic layers with homogeneous silver doping are obtained.
 2. Theprocess according to claim 1, wherein the stoichiometric excess of leadoxide is 1 to 3 mol%.
 3. The process according to claim 1, wherein, toobtain a predetermined thickness of 2-4 μm for the electrode layersafter the step of sintering, the step of applying electrode layersincludes determining a layer thickness ratio of the piezoceramic layersto the electrode layers and to a dopant content of the rare earth metaland then selecting a layer thickness allowance which compensates forsubsequent layer thickness loss by the inward diffusion of the silverinto the piezoceramic layers.
 4. The process according to claim 1,wherein the rare earth metal is selected from a group consisting oflanthanum and neodymium.
 5. The process according to claim 1, whereinthe piezoceramic powder is a piezoceramic powder which has a complexB-site doping.